1. Field of the Invention
The present invention relates to a multifilamentary superconducting wire employing an oxide superconductor and an oxide superconducting coil which is applied to a high temperature superconducting magnet or the like, and more particularly, it relates to a wire and a coil which can exhibit excellent superconductivity under a high magnetic field.
2. Description of the Background Art
Due to discovery of yttrium, bismuth and thallium based oxide superconducting materials respectively exhibiting critical temperatures (Tc) of 90 K, 108 K and 125 K which are higher than the liquid nitrogen temperature of 77.3 K, application of such materials to energy and electronics fields has been expected. Under such circumstances, formation of a wire by an oxide superconductor (ceramics superconductor) has been positively developed for application to the energy field, since the discovery of this material.
For example, European Patent Laying-Open No. 0 609 920 A2 discloses a multilayer superconducting wire comprising oxide superconductor and metal layers which are alternately stacked with each other. In manufacturing of this wire, raw material powder for the oxide superconductor is mixed with a binder. The mixture as obtained is molded into a sheet, for example. The sheet as obtained is superposed with a silver sheet and spirally wound to obtain a rod. This rod is heated and charged in a silver pipe, and then subjected to hot isostatic pressing (HIP), for forming a wire. This wire is rolled into a plate material, which in turn is heat treated.
In manufacturing of an oxide superconducting wire, on the other hand, a solid phase reaction method called a powder-in-tube process is suitable for mass-producing long superconducting wires, due to employment of a technique of working an ordinary metal wire such as wire drawing or rolling. Among superconducting wires manufactured by this solid phase reaction method, a wire exhibiting a relatively high critical current at 4.2 K under a high magnetic field exceeding 20 T, for example, in relation to a silver sheath wire employing a Bi-based 2223 phase, in particular. Further, properties at 20 K are also examined (refer to Sato et al., Appl. Phys. Lett. 61(6), Aug. 10, 1992, pp. 714-716).
The technique of manufacturing a silver sheath wire by the solid phase reaction method is applied to a high temperature superconducting coil. While the temperature for using a conventional superconducting coil employing an alloy-based superconducting wire is limited to not more than 4.2 K, employment at a higher temperature can be expected in a superconducting coil formed by winding the aforementioned silver sheath wire.
As to an important technique for manufacturing a superconducting coil, it is necessary to design the coil so that the coil itself can withstand high electromagnetic stress which is generated in its interior upon generation of a high magnetic field.
In relation to such a silver sheath wire, silver forming the matrix is generally regarded as a reinforcing material. Thus, the wire is so formed that the silver occupies a high ratio in its section. When the wire is worked into a coil, however, reinforcement by silver is limited. Therefore, it is an important subject to obtain a coil which can sufficiently withstand electromagnetic stress from the silver sheath wire.